Galactic cosmic ray modulation and interplanetary medium perturbations due to a long‐living active region during October 1989

Abstract

During October 1989, three very energetic flares were ejected by the same active region at longitudes 9° E, 32° W, and 57° W, respectively. The shape of the galactic cosmic ray variations suggests the presence of large magnetic cloud structures (Nagashima et al., 1990) following the shock‐associated perturbations. In spite of long data gaps the interplanetary observations at IMP 8 (near the Earth) and ICE (∼1 AU, ∼65° W) confirm this possibility for the event related to the 9° E flare; the principal axes analysis shows that the interplanetary magnetic field variations at both spacecraft locations are mainly confined on a meridian plane. This result suggests that the western longitudinal extension of this cloud is indeed very large (≥75°). The nonnegligible depression in the cosmic ray intensity observed inside the possible cloud related to the 57° W flare indicates that also the eastern extension could be very wide. The analysis of neutron monitor data shows clearly the cosmic ray trapping effect of magnetic clouds; this mechanism seems to be responsible for the enhanced diurnal effect often observed during the recovery phase of Forbush decreases. We give an interpretation for the anisotropic cosmic ray peak occurring in the third event, and, related to that, we suggest that the Forbush decrease modulated region at the Earth's orbit could be somewhat wider than the magnetic cloud, as already anticipated by Nagashima et al. (1990). By this analysis, based mainly on cosmic ray data, we show that it is possible to do reasonable inferences on the large‐scale structure of flare‐related interplanetary perturbations when interplanetary medium data are not completely present.